Add smooth transition between external perimeter and small perimeter speeds

[murk-sy]

Is your feature request related to a problem? Please describe.
This is a fairly minor problem, but:
On thin and tall models with either width changing spikes or, as shown, curvy shapes, the transition between full External Perimeter Speed (ExPS) and Small Perimeter Speed (SmPS) is visible. It makes the part that is otherwise flawless look glued together. Glued together extremely well, but still.

Describe the solution you'd like
Ideally, SmPS should not be used somehow in cases like shown, or there should be a transition between speeds.

Describe how it would work
Assuming ExPS being 40 and SmPS being 25 at radius 6.5mm (as noted in description), the speed should smoothly interpolate between 40 at radius 10 mm, 32.5 at radius 8.25 mm and 25 at radius 6.5mm (though making it lower - or settable - might be a good idea).
If printing a single object with SmPS set to same as ExPS, this transition is already made somewhat smooth by auto cooling, but that does not work with multiple objects printed.
Max Volumetric slope of 2 is already used as well.

Describe alternatives you've considered
I could just manually change the SmPS setting to same as ExPS, but I don't know how this would affect really small holes - this model in particular doesn't have them.

Additional context
An example model for testing:

Small perimeter speed test 2022-10-07.zip

[rtyr]

Similar to #2785.

[aknezevi-work]

I honestly don't know why small perimeter speed overrules external perimeter speed. Small perimeter speed should only be used for inner perimeters, that is where it is actually useful.

[murk-sy]

Just as an update, the Cooling Logic Enhancements: "Consistent Surface" in 2.9.3-beta1 does not seem to resolve the issue if small perimeter speeds are small enough.

Single parts shown mostly as example since they're constrained by minimum speed too, issue is more obvious with multiple parts.

Single part (MiniIS 0.20 Structural, speed view)

Single part with Consistent surface enabled

Multiple parts (small perimeter speed set to 20)

Multiple parts with Consistent surface enabled (small perimeter speed set to 20)

Inside view, infill could clearly be slowed down to speed up external surfaces:

Or would sort of the official recommendation be to use the same setting for small perimeters are external perimeters?

[aknezevi-work]

The culprit here is:

PrusaSlicer/src/libslic3r/GCode.cpp

Lines 3170 to 3188 in 69c8e56

	if (perimeter.extrusion_entity->length() <= SMALL_PERIMETER_LENGTH)
	speed = m_config.small_perimeter_speed.get_abs_value(m_config.perimeter_speed);
	gcode += this->extrude_smooth_path(perimeter.smooth_path, perimeter.extrusion_entity->is_loop(), comment_perimeter, speed, perimeter.wipe_offset);
	this->m_travel_obstacle_tracker.mark_extruded(
	perimeter.extrusion_entity, print_instance.object_layer_to_print_id, print_instance.instance_id
	);
	const bool is_extruding{
	!perimeter.smooth_path.empty()
	&& !perimeter.smooth_path.front().path.empty()
	&& perimeter.smooth_path.front().path.front().e_fraction > 0
	};
	if (
	!m_wipe.enabled()
	&& perimeter.extrusion_entity->role().is_external_perimeter()
	&& m_layer != nullptr
	&& m_config.perimeters.value > 1
	&& is_extruding

The solution seems to be is to put "&& !perimeter.extrusion_entity->role().is_external_perimeter()" to line's IF statement:

PrusaSlicer/src/libslic3r/GCode.cpp

Line 3170 in 69c8e56

if (perimeter.extrusion_entity->length() <= SMALL_PERIMETER_LENGTH)